Components in the hot gas path of gas turbines, such as rotating blades and stationary vanes, have been produced by metal casting. Cooling passages can be formed in the component by placing pre-formed inserts in the mold. Such inserts are typically made from oxide ceramics. A high-temperature material such as ceramic is required to withstand the molten metal temperatures during pouring. After solidification of the casting the inserts are removed. This can be done for example by dissolving them with a caustic solution. However, ceramics are brittle. If an insert is too thin it can break during pouring or solidification of the metal, resulting in a defective component. This is especially true of directionally solidified (DS) and single crystal (SX) cast components, in which the mold is withdrawn slowly from the furnace during solidification, which increases the amount of tine that the insert is exposed to the molten metal.
Pre-formed ceramic inserts are expensive, and are limited as to the size and complexity of cooling passages that can be produced. Small passages or tortuous circuits would require inserts that are thin, and therefore delicate. In some turbine airfoils and other components it is desirable to provide numerous small cooling channels just below the component surface. One approach is first to cast an airfoil core structure of a superalloy. Grooves are then formed in the outer surface of the core, either during casting or by machining. A superalloy skin is formed separately and bonded to the core, covering the grooves to form sub-surface cooling passages. Transient liquid phase (TLP) bonding has been used for this, in which a foil, powder, or tape, made of a low melting point variant of the airfoil core material is placed between the skin and the core. Upon heating, the TLP partially melts, fusing the skin to the core. This process can use a smaller gap and a higher temperature than soldering or brazing. The TLP material contains melting point depressants such as silicon and/or boron. However, the size of skin that can be accurately formed and bonded has been a limitation of this process.
The assignee of the present invention has invented a process for forming the skin in place on the metallic core, as described in U.S. Pat. No. 5,875,549. Grooves may be formed in the surface of the core. A fugitive filler material such as a ceramic paste is placed in the grooves. The fugitive material is smoothed flush with the core surface, and then a skin of metal is deposited on the surface of the core and the fugitive material. The skin bonds to the core surface between the grooves. The fugitive material is removed, leaving subsurface cooling channels.
However, even with a diffusion bonding heat treatment, the bond interface between the skin and core is weaker than the material of the skin or the core because no grain growth occurs across the bond line. A stronger bond interface is desired.